The current composer.json values for Symfony packages prevent this project being compatible with Symfony 5.

"symfony/config": "~2.4|~3.0|~4.0",
"symfony/dependency-injection": "~2.4|~3.0|~4.0",
"symfony/http-kernel": "~2.4|~3.0|~4.0",

To allow for testing ~5.0 could be added to the above three Symfony components which should be all that's needed.